lib: sbi: Narrow vendor extension range

The vendor extension ID range is large, but at runtime at most
a single ID will be available. Narrow the range in the
register_extensions callback. After narrowing, we no longer
need to check that the extension ID is correct in the other
callbacks, as those callbacks will never be invoked with
anything other than the single ID.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>